Betaines have established themselves in recent years in the cosmetic industry as a firm constituent of formulations, in particular, for cleansing the hair and body. Betaines are able to form a dense and creamy lather which remains stable over a long period even in the presence of other surfactants, soaps and additives. Moreover, betaines have recognized good cleaning properties without any irritative side-effects, even for sensitive skin.
The preparation of betaines is described in detail in the relevant patent and specialist literature (see, for example, U.S. Pat. No. 3,225,074). In general, compounds containing tertiary amine nitrogen atoms are reacted with ω-halocarboxylic acids or salts thereof in aqueous or hydrous media.
The compounds containing tertiary amine nitrogen atoms used are, in particular, fatty acid amides of the general formula (I)R3—CONH—(CH2)m—NR4R5  (I)in which    R3 is an alkyl radical of a fatty acid which is optionally branched and can optionally comprise multiple bonds, optionally hydroxyl groups,    R4, R5 are identical or different alkyl radicals having 1 to 4 carbon atoms, and    m may be 1 to 3.
The alkyl radical R3 can be derived from natural or synthetic fatty acids having 6 to 20 carbon atoms, preferably from natural vegetable or animal fatty acids having 8 to 18 carbon atoms, and their naturally occurring or specifically adapted mixtures with one another or among one another.
Suitable fatty acids are, for example, caproic acid, caprylic acid, capric acid, lauric acid, palmitic acid, stearic acid, behenic acid, linoleic acid, linolenic acid, and ricinoleic acid.
Preference is given to the naturally occurring fatty acid mixtures with a chain length of from 8 to 18 carbon atoms, such as coconut fatty acid or palm kernel fatty acid, which can optionally be hydrogenated by suitable hydrogenation methods.
These fatty acids and fatty acid mixtures are reacted by customary condensation reaction in a first process stage at 140° to 200° C. with amines of the general formula (II)H2N—(CH2)m—NR4R5  (II)in which R4 and R5 and m have the meaning specified in the formula (I), to give the fatty acid amides with tertiary nitrogen atoms of the general formula (I).
The subsequent quaternization reaction to give betaines of the formula (III)R3—CONH—(CH2)m—N+R4R5(CH2)yCOO−  (III)in which R3, R4, R5 and m have the same meaning as in the formulae (I) and (II) and y may be 1, 2 or 3, is carried out according to the process known in the literature as the second process stage.
The process usually involves adding ω-haloalkylcarboxylic acids, preferably chloroacetic acid or the sodium salt of chloroacetic acid to the fatty acid amide of the formula (I) in the aqueous medium and completing the quaternization in a reaction for several hours at about 80° to 100° C. Depending on the fatty acid or fatty acid mixture used, in order to maintain stirrability, a minimum amount of water must be present as the reaction progresses. The standard commercial solids concentration of the betaine solutions prepared in this way is therefore about 35 to 37% by weight or below.
However, to reduce storage and transportation costs and for formulation-related reasons during further processing, in many cases a higher concentration is strongly desired. In the past, a series of processes to fulfill this requirement have therefore been proposed. The solids content of these mostly highly viscous solutions could, however, only be increased to about 50%, and in the case of commercial highly concentrated betaines, it is mostly about 45%, the active betaine content thus being about 35%.
DE-C-3 613 944 discloses a process in which the quaternization is carried out in an organic polar solvent with a water fraction of 20% by weight and some, or all, of the solvent is removed by distillation and then the desired concentration is established again using an industrially usable solvent. Apart from the fact that the process is technically complex and cost-intensive, organic solvents and also the residual contents, which cannot be removed by distillation during further processing, are often undesired in cosmetic formulations.
Although the process according to DE-C-3 826 654 contains an example without an organic solvent, the viscosities obtained are too high for industrial production. Furthermore, relatively large amounts (3 to 20% by weight) of a nonionic surfactant (polyoxyethylene ether with 10 to 250 oxyethylene units) are required as a liquefier. Added amounts, in this order of magnitude, are undesired since they can adversely affect the end formulations in physical and/or physiological terms.
DE-C-4 207 386 describes highly concentrated betaines which comprise 1 to 3% by weight of saturated or unsaturated fatty acid as a thinning principle. Betaines which are prepared by this method can be concentrated up to a solids content of 48% by weight.
Similar concentrations are obtained according to EP-B-1 140 798. The thinning principles here are glutamic acid (salts) and analogous amino acids, which are added to the quaternization reaction.
DE-C-19 505 196 describes the use of sulfobetaines, amphoglycinates, trimethylglycine or dicarboxylic acid (diamides) in concentrations of from 4 to 8% by weight during the carboxymethylation. The betaines obtained have a solids content of about 50% by weight and an average viscosity.
The solids content is likewise 50% by weight for betaines which have been prepared according to DE-A-19 700 798 using up to 5% by weight of polybasic optionally hydroxy-functionalized carboxylic acids during the carboxymethylation.
Similar solids contents are achieved by adding 0.05 to 2% by weight of cyclodextrins or dextrans, as described in DE-A-10 207 924.
The use of 0.5 to 5% by weight, preferably 2 to 4% by weight, of betaines from short-chain mono- or dicarboxylic acids for diluting cocobetaine is described in EP-C-656 346. Here, betaines with a solids concentration of 45% by weight are obtained.
DE-C-4 408 183 describes betaines with up to 54% by weight of solids. The betaines comprise 1 to 10% by weight of a hydroxycarboxylic acid. In order to obtain flowable products with more than 50% by weight of solids, about 5% by weight of citric acid is required. The betaine content is then about 32% by weight.
DE-C-19 523 477 describes the use of polyfunctional carboxamides, e.g., adipic acid diamidamine, during carboxymethylation. Although the described betaine concentrates comprise 60% by weight of solids, the betaines are no longer of low viscosity. Since during the carboxymethylation a viscosity maximum is passed through, industrial preparation of these products is very costly and complicated unless large amounts of adipic acid betaine are added to the betaine, which, in turn, strongly influences the properties of the betaine.
Solids contents of about 50% by weight are also obtained according to U.S. Pat. No. 6,683,033. Here, 0.5 to 3.5% by weight of phosphoric esters of optionally low-ethoxylated fatty alcohols and/or dimer acids are used as liquefiers. The betaine content of the examples is below 37%.
DE-C-4 408 228 describes betaines with solids contents above 50% by weight comprising 1 to 10% by weight of a nonionic surfactant with HLB 6 to 12 and/or hydroxy-carboxylic acids, 1 to 6% by weight of polyols and optionally 1 to 10% by weight of fatty acid (salts). For high solids contents, in most cases greater than 6% by weight of additives are required, which is undesirable with regard to potentially negative effects on formulations. The actual betaine content is only about 32% by weight.
Due to the high storage and transportation costs relative to the price of the product, there is a need for even more highly concentrated flowable and pumpable aqueous solutions of betaines which are free from lower alcohols such as, for example, methanol, ethanol, propanol or isopropanol.